Previously the vortex flow blower has usually been provided with blades formed radially in the impeller. Since the vortex flow blower exhibits an advantage in that high wind pressure can be obtained with reduced size, a variety of disclosures and studies have been made for the purpose of improving the above-identified advantage.
For example, a study is disclosed in Transaction of Japan Machinery Society, Vol. 45 (published in August 1979), P. 1108-1116. According to the study, the characteristic (i.e., the characteristic about the relationship between discharge flow rate and discharge pressure) of the vortex flow blower is changed by changing the ratio R.sub.1 /R.sub.2, where R.sub.1 represents the radius of a circle connecting the inner end of a blade and the axial center and R.sub.2 represents a radius connecting the outer end of the blade and the axial center. According to this it is disclosed that both the flow rather coefficient and the pressure coefficient are higher when the value of R.sub.1 /R.sub.2 is 0.68 than when it is 0.82, and they become higher when the value is 0.75. In the vortex flow blowers which have been put into practical use, the smallest value of R.sub.1 /R.sub.2 is about 0.68.
Although R.sub.2 must be a small value for the purpose of reducing the size of the vortex flow blower, the following problems arise; namely, the value of R.sub.1 /R.sub.2 must be decreased when the desired flow rate is satisfied with a reduced size of the vortex flow blower since the flow rate significantly depends upon the value of R.sub.2.sup.2 (1-R.sub.1 /R.sub.2). However, if the value of R.sub.1 /R.sub.2 is reduced to 0.75 or less, the pressure coefficient becomes smaller as described above. Furthermore, since the outer radius R.sub.2 has been reduced, peripheral speed u.sub.2 at the outer radius R.sub.2 is also lowered, thereby causing the discharge pressure to be excessively lowered since the pressure characteristic is determined by the product of the pressure coefficient and the square of u.sub.2. Therefore, R.sub.2 must be a small value, and R.sub.1 /R.sub.2 must be a small value and the pressure coefficient must be significantly increased in order to reduce the size of the vortex flow blower.
When improved characteristics are desired without any change in the size of the vortex flow blower, the following problem arises; namely, if the value of R.sub.1 /R.sub.2 is increased to about 0.75 for the purpose of improving the pressure performance in the case where the value of R.sub.1 /R.sub.2 is constant, the flow rate is inevitably reduced and, on the contrary, if the value of R.sub.1 /R.sub.2 is reduced for the purpose of increasing the flow rate, the pressure coefficient is lowered. Therefore, when an improved characteristic is desired without changing the size of the vortex flow blower, R.sub.1 /R.sub.2 must be reduced and the pressure coefficient must be increased.
Vortex flow blowers designed to improve their aerodynamic performance are disclosed, for example, in Japanese Patent Unexamined Publication No. 50-5914 and Japanese Patent Unexamined Publication No. 61-155696, each of which is provided with an impeller formed in such a manner that only the axial inlet angle and the exit angle of its blade are inclined at a certain angle which is respectively smaller or larger than 90 degrees. Furthermore, vortex flow blowers, although their objects are unclear, are disclosed in Japanese Utility Model Examined Publication No. 55-48158 and Japanese Utility Model Unexamined Publication No. 56-85091, each of which is provided with an impeller formed in such a manner that both or one of the inlet angle and the exit angle in the circumferential direction of its blade are or is inclined at a certain angle which is different from 90 degrees.
Further, a method of manufacturing an impeller is disclosed in Japanese Patent Unexamined Publication No. 51-57011, and according to this method the impeller is composed of two pieces divided in its axial direction in order to make a core unnecessary when forming the impeller from a casting, and the thus divided two pieces are coupled to each other afterwards.
Since the vortex flow blower exhibits an advantage in that it can serve as a clean air source with a reduced size, it has recently been recently widely used. Therefore, there arises a desire for the vortex flow blower which is capable of generating higher wind pressure and whose size is reduced with the discharge pressure maintained as it is. However, in the conventional technologies including the above-described technologies, only one of the exit angle in the circumferential direction, the inlet angle and the axial angle of the blade is taken into consideration and the shape of the blade is not formed so as to be adapted to the three dimensional internal flow which takes place inherently in the vortex flow blower, so that turbulence of internal flow such as swirls and stagnation cannot be prevented. Therefore, the following problems arise that it is difficult to further reduce the size of the vortex flow blower and a predetermined pressure maintained, and it is difficult to obtain higher discharge pressure with the flow rate maintained without enlarging the size of the vortex flow blower.
Furthermore, since the conventional vortex flow blower have been insufficient in terms of noise reduction, they cannot be used as medical equipment or the like which are used in quiet environments.
In addition, according to the method of manufacturing an impeller disclosed in Japanese Patent Unexamined Publication No. 51-57011, it is difficult to manufacture an impeller blade having a three dimensional shape.
Furthermore, when the impeller is manufactured by a low pressure casting process, since there are problems of run or fluidity it is difficult to reduce thickness to the blade. Therefore, it is difficult to reduce the secondary moment of inertia of the impeller, thereby causing starting torque when starting the impeller and, as a result, the size of the motor cannot be reduced.
Furthermore, the metal mold used when the impeller is manufactured by an integral molding process such as die-casting or chill-casting process is expensive, so that it is difficult to inexpensively manufacture an impeller having different aerodynamic performance.